Biostratigraphic Analysis of the Cretaceous Abeokuta Group in the Eastern Dahomey Basin, Southwestern Nigeria

Abstract

Shales of the Abeokuta Group in the Dahomey Basin yielded abundant foraminiferal tests. The study investigated sediments from three wells that were drilled for petroleum exploration: Well – X located in the offshore deep water, Orimedu-1 and Ise-2 in the coastline region to establish their Cretaceous biostratigraphic units through planktonic foraminiferal zonation. Calcareous benthonic foraminifers occur abundantly and consistently throughout the sequences in the three wells while planktonic foraminifera are more abundant in the Well X than those in the Orimedu-1 and Ise-2.The identified planktonic foraminiferal species are: Rotalipora greenhornesis, Rotalipora cushmani, Dicarinella primitiva, Praeglobotruncana helvetica, Marginotruncana sp. Heldbergella delrionensis, Helbergella planispira, Hedbergella simplex, Whitenella inonata, Whitenella baltica, Globotruncana aegytiaca, Globotruncana sp., Heterohelix reymenti, Heterohelix moremani, Heterohelix globulosa, Abathomphalus mayaroensis and occurrence of Orthokarstenina oyea, Ammobaculites corprolithiformis, Ammobaculities sp and Bolivina sp. Calcareous benthonic foraminiferal species were more abundant than the planktonic species in Orimedu-1 and Ise-2 wells. The benthonic species decrease drastically in the well-X offshore deep water where the planktonic species were most abundant. This trend may suggest an increasing water depth from near coastline to offshore regions in the Cretaceous times.Six planktonic foraminiferal zones were recognised across the three exploratory wells. These include Rotalipora greenhornesis (middle – late Cenomanian), Helvetoglobotruncana helvetica (early – late Turonian), Marginotruncana pseudolinneiana (late Turonian), Marginotruncana renzi (Coniacian – Santonian), Globotruncana calcarata (late Santonian – early Campanian) and Globotruncana aegytiaca (Maastrichtian) Zones. The Rotalipora greenhornesis and Praeglobotruncanae helvetica Zones (Cenomanian – Turonian) were correlated with the standard zones in the Tethyan regions.The Well-X indeed penetrated older Cenomanian to Turonian intervals in the investigated wells, thereby confirming the presence of older marine sequences in the Eastern Dahomey Basin contrary to previous suppositions.